If President Barack Obama and the U.S. government consider the perpetrator of the Boston bombings a terrorist, then why not the shooter who killed 20 children and six adults at Sandy Hook Elementary School?

“Based on the evidence at this point, is there any difference between Sandy Hook and Boston other than the choice of weapon?” Sen. Claire McCaskill (D-Mo.) asked Homeland Security Secretary Janet Napolitano during a congressional hearing Wednesday.

Napolitano didn’t have a simple response.

“In terms of intent for death and destruction and injury, no,” she said, “Methodology, yes. And we don’t know the motivation behind, certainly, Boston — we don’t know whether it was domestic, it’s international ...”

“Or if it was identical to the motivation in Sandy Hook,” McCaskill suggested.

“It’s impossible for me to sit at the table today and say they are identical except in effect and impact,” Napolitano said.

On the same day the Senate was expected to vote on several gun-control provisions, McCaskill pressed Napolitano to reevaluate when and how the federal government defines a criminal act as terrorism — especially when, in the case of the Boston incident, no suspects or motives are known.

“We are so quick to call Boston terror,” McCaskill said. “Why aren’t we calling the man with the high-capacity assault weapon and the high-capacity magazine, why aren’t we calling him a terrorist?”

“I don’t know the answer to that question,” the secretary replied.

“It just is troubling to me,” said McCaskill, a former county prosecutor. “I think both of them, maybe they had identical motives. Just one chose a military-style weapon with a high-capacity magazine, and the other one chose to make a homemade bomb.”

A day after Monday’s twin Boston bombings that killed three people and injured 176, Obama told reporters the FBI is investigating the attack as “an act of terror.” Napolitano repeated that line in testimony before the Senate Homeland Security Committee on Wednesday morning.

Later Wednesday afternoon, an amendment by Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) — requiring background checks for firearms purchases — was expected to fall short of the 60 votes needed to pass.

The Senate will take up eight other amendments as well, including a ban on assault weapons and a ban on high-capacity magazines like those used by Adam Lanza in the Sandy Hook shooting in Newtown, Conn., in December.